1. Field of the Invention
This invention relates generally to oil and gas well tubular casing and more particularly has to do with the construction of high pressure casing joints operable when made-up and run into a well to withstand extreme fluid pressures and for sealing off thereof to prevent escape of high pressure fluid or gas through the tubular casing assembly at the joints.
2. Description of the Prior Art
The search for oil and gas reserves has brought about the exploration of ever deeper formations. These deeper formations require longer strings of production pipe, casing and liners used in the exploration and production of oil and gas. Such wells may be subject to extremely high pressures from formation zones. The increased length of tubular strings imposes the upper portion of the string to very high tensile loads and where high pressure exists from deeper formations may also expose the upper portion of the string to high internal pressures where there is little or no off-setting external pressure on the casing or tubing. With standard joints there are limitations to the depths to which a string of casing or tubing can be run.
Casing joints are known which are reliable and versatile and combine excellent annular clearance with optimal tensile strength and three positive metal-to-metal seals for many casing applications. Such joints have been commercially available for some time and are sold under the mark "Triple Seal" by the Hydril Company, the assignee of the present invention and application. Such "Triple Seal" joints have featured three positive metal-to-metal seals comprising a fourteen degree external seal, a middle ninety degree torque shoulder and seal and a fourteen degree pin to box internal seal. Such joints have been provided on casing members which have been formed and stressed relieved. The joints have been offered as two-step cylindrical threaded surfaces with modified API buttressed threads. The joint sold under the "Triple Seal" mark has featured a ninety degree torque shoulder intermediate the two steps of the thread profile.
A new generation thread design sold under the mark "Triple Seal" or "MAC" has featured a reverse angle intermediate torque shoulder between the two steps where the reverse angle is a positive angle with respect to a radius through the torque shoulder. The "MAC" connection has featured dissimilar angles on the reverse angle torque shoulder.
There has developed a need for even more performance from a "Triple Seal" type casing connection. It is therefore an object of the invention to provide a casing connection or tubular joint which may be provided on a standard upset of the end of tubular members which has very high joint efficiency.
It is another object of the invention to provide protection against damage in the field of the external seals of the connection.
It is a further object of the invention to provide locking of the intermediate positive angle torque shoulder with the threads on either side of the combination torque shoulder and sealing surface.
It is a further object of the invention to provide facing shoulders on the end of the pin and the interior end of the counterbore of the box whereby under extreme internal pressures, the box traps the end of the pin causing the box and pin to flex together.